The present invention relates to a pneumatic control system for an automobile automatic transmission of the type having a mechanical planetary gear train with hydraulically actuated gear shift elements for shifting of the transmission elements. A manual control valve, dependent upon the position of the transmission gear select lever (hand operated), conveys a pressure medium when one of the forward gears is engaged for actuating a gear shift element (forward gear clutch) associated with the gear.
In automatic hydrodynamic-mechanical transmission, the turbine of the hydrodynamic torque converter is normally connected positively with the drive gears, when one of the forward gears is selected, by a so called forward gear clutch. Even during idle, however, the engine transmits hydraulic energy through the convertor pump to the turbine wheel so that a moment is produced which causes the vehicle to crawl, i.e. to travel slowly forward even though the accelerator pedal is released. Usually, then, in the case of automatic transmissions, the brake must be employed to keep the vehicle stationary or the gear select lever must be moved in the neutral position.
Crawling of the vehicle during idling may also be eliminated by providing a special clutch arranged between the turbine wheel and engine actuated by an electrical idling switch. An additional clutch of this type, however, requires considerable space and is costly.